There are several aspects to John’s hypothesis, so it was quite a challenge to condense the argument into just three pages of the book (and even more of a challenge to use only simple words to describe some rather abstruse concepts; I lack Roberto Trotta‘s ability to illustrate rarefied ideas with monosyllables).

One aspect is relatively easy to describe and understand: John argues that advanced civilisations will collapse. Note that this is not collapse in the sense of Gibbon’s Decline and Fall! Rather, civilisations will develop in an inward direction rather than outward into space. Qualities that might be used to characterise civilisations – their use of space, time, energy, matter – will all exhibit an increase in density. In John’s words, civilisations will undergo STEM compression.

You can argue that STEM compression is happening to our own civilisation. Let’s consider just one of those four STEM dimensions. In the past, the majority of humans lived in small settlements and the density of information was low. Many people now live in cities, and anyone visiting London or Berlin or New York will immediately appreciate the greater information density to be found in these places compared to that in hamlets or villages. Increasingly, people in developed nations work for knowledge-based companies that possess levels of information density exceeding those of cities. The suggestion is that the density of all four quantities will increase as civilisations become more advanced. And the logical end of this STEM compression? Well, the physical limit is set by the Planck scale. A black hole is thus the natural end of an advanced civilisation. It will be the natural end of our own civilisation.

One commendable aspect of the transcension hypothesis, at least to my mind, is that it offers specific and potentially falsifiable predictions – effects that astrophysicists could look for. (Read the book, or even better read John’s original paper, to learn more about those predictions.) However, is it reasonable to suppose that civilisations inevitably take a developmental path that leads to transcension? Well, John provides support for the idea by arguing from “evo-devo”, or evolutionary developmental biology. Evolution is a random process; development, though, is directed and constrained – a cat embryo gives rise to a cat, a dog embryo gives rise to a dog. Both evolution and development play important roles in life. What if the universe is engaged in a life cycle? (It was born in a Big Bang, it has grown as it aged, and there are processes relating to black holes that might allow it to spawn new universes.) If we live in an evo-devo universe then perhaps transcension is inevitable – just as an embryo gives rise to grown animal.

There are too many “if’s” in the argument for me to buy thetranscension hypothesis. However, in criticising the hypothesis I wrote that it requires that “all individual elements in all civilizations in all neighbouring galaxies develop in the same way”. This was one of those occasions where trying to compress information into a few lines (itself a symptom of STEM compression?) distorts the meaning. John sent a rebuttal to this, and rather than paraphrase it I’ll simply present it here:

Developmental processes are a very small subset of living processes. They certainly don’t comprise anything like “all individual elements” in an organism, and they ensure that evolutionary diversity grows over time within that organism, both within and across life cycles. If we live in an analogously evo devo universe, only a small subset of average observable changes in any civilization would be developmental. And those changes would be there to ensure that evolutionary diversity grows among civilizations over time, which is perhaps the main reason why we might have multiple civilizations and intelligent planets in our universe, if each is an incomplete and finite computational system.

I believe it will be worthwhile to examine the transcension hypothesis in more detail. Unlike so many “solutions” to the Fermi paradox, this one offers avenues for further research.

In the second edition of Where is Everybody? I discuss what Gerard Foschini calls the canonical artefact (TCA) — a flag for the presence of an advanced intelligent life-form.

I don’t propose to discuss the details of TCA in this post — you can read the book if you’re interested — but I do want to provide an update. I was fairly sure that no-one had constructed an example of TCA, but yesterday Foschini emailed me a photo of it: he built it out of coin stacks with black rubber test tube stoppers as separators. Below, shown with gratitude, is Foschini’s TCA.

The search for extraterrestrial intelligence (SETI) has traditionally focused on looking for messages, encoded in radio waves or laser pulses, that aliens might have sent our way. There is, however, another approach to SETI: we could look for evidence of activity that would allow us to deduce the presence of intelligence. The detection of an alien message aimed directly at Earth’s inhabitants would undoubtedly be more exciting than the discovery of, say, exhaust from a distant antimatter rocket – but the key conclusion would be the same: we would know that we were not alone.

In 1960, Freeman Dyson pointed out that the development of technologically advanced civilisations would be limited by energy considerations: growing civilisations require access to increasing amounts of energy. Dyson suggested that a sufficiently advanced civilisation would eventually need to harvest the entire energy output of its home star – which it could achieve by dismantling a planet and using the resulting material to create a host of solar collectors. Such a star-enshrouding swarm came to be known as a Dyson sphere. A galaxy-spanning civilisation – a so-called Kardashev type-3 civilisation – would do this for all the stars in its galaxy.

A Dyson sphere around the Sun. The sphere would not need to be a solid object; a “Dyson swarm” is perhaps more realistic. (Public domain)

It would be difficult to hide such astroengineering activity. As Dyson himself pointed out, the laws of thermodynamics mean that a Dyson sphere could be detected by the glow of its radiated waste heat. Visible starlight would be replaced by radiation that peaked in the mid-infrared. If the creation of Dyson spheres is indeed a common activity then the signature of a type-3 civilisation would be a galaxy dim in the visible part of the spectrum but bright in the mid-infrared part of the spectrum. In other words, we can do SETI by looking for the absence of light combined with the presence of waste heat.

A Dyson swarm. (Public domain)

Jason Wright, an astronomer at Pennsylvania State Universe, and his colleagues have recently taken precisely this approach to SETI. They wrote software to assess the 100 million objects contained within the catalogue generated by NASA’s WISE (Wide-field Infrared Survey Explorer) mission. Catalogue objects that were merely mission artefacts, or were clearly not galaxies, were rejected by hand. The team then began looking for optically dim and infrared bright objects. They ended up with a target group of about 100,000 galaxies emitting large amounts of radiation in the mid-infrared. None of those 100,000 galaxies contained a civilisation that was reprocessing more than 85% of its starlight into the mid-infrared: there are no galaxy-spanning type-3 civilisations in our cosmic neighbourhood. Only 50 of those 100,000 galaxies possessed a spectrum that was consistent with the reprocessing of 50% of its starlight into the mid-infrared; but of those few galaxies the infrared brightness can almost certainly be explained by natural processes. So although Wright and his colleagues found a few objects that might be of interest to SETI scientists (and are certainly of interest to traditional astronomers), in essence they registered a null result.

Now, you could argue that we have no idea of the choices that technologically advanced civilisations might make. Perhaps they won’t build Dyson spheres. Well, that’s surely the case. Perhaps they use rotating black holes or antimatter annihilation or even something we don’t yet understand for power generation. Nevertheless, any astroengineering that takes place on a galaxy-wide scale is going to generate waste heat. Wright and his team were looking for signs of that waste heat and they found nothing.

Traditional SETI has so far failed to find a civilisation in our Galaxy at the type-1 or type-2 stage. This recent result from Wright and his colleagues suggests that type-3 civilisations anywhere are rare or non-existent. What are we to make of these null results? Well, perhaps our understanding of the development of technological civilisation is flawed; perhaps aliens are “green” and their long-term survival requires a more sustainable approach to energy use. Perhaps. But there’s another, more obvious explanation: we are alone in the universe.

Early on in his book Lucky Planet, David Waltham writes about the planet-detecting capabilities of the Kepler space mission. When discussing the chances of Kepler finding a truly Earth-like planet – a body that’s the same size as our Earth and orbiting in the habitable zone of its stellar system – he states that the “announcement of such a world could well come between me finishing this book and its publication (an occupational hazard of discussing topical subjects)”. He was spot on. At about the same time Lucky Planet hit the bookshops, NASA announced the discovery of Kepler-186f, a planet whose radius is about 10% greater than Earth’s and that sits towards the outer edge of the habitable zone of its dwarf star.

Let’s face it. It’s likely that there are billions, perhaps trillions, of Earth-like stars in the cosmos. Can we plausibly argue that our Earth is somehow special, that it alone of all similar worlds possesses complex (which is to say multicellular) life?

Well, yes we can. With the limited data available to scientists we can’t say with any degree of certainty that Earth is the only planet that hosts complex life. But we can certainly make the argument that Earth is exceptional – and Waltham does so beautifully.

Lucky Planet gives the most accessible treatment I’ve yet read of the anthropic selection effect. Our planet has enjoyed four billion years of clement weather, and this climate stability has surely been key to life’s continued existence. Earth’s climate could easily have followed a trajectory towards ice or fire, turning the planet into a snowball or a boiling hell; the climate could have done rapid flip-flops between periods of frigidity and periods of heat. Instead, the average surface temperature of our planet has seen a gentle cooling trend on top of which are relatively minor fluctuations, measured in tens of degrees. With this climate life has been able to thrive.

We can explain life’s continuing existence by saying that life is robust, resilient, capable of withstanding any of the shocks that Nature throws its way. Or we can go even further and argue that various feedback mechanisms allow life itself to create, maintain and develop the conditions needed for life to survive and thrive. This is the Gaia hypothesis. Combine this approach with the vast number of Earth-like planets that exist and one surely must conclude that the Galaxy is teeming with life.

But we can just as well explain life’s continuing existence by attributing it to luck. As Waltham points out, the anthropic selection effect means that intelligent observers must find themselves on planets on which the past climate was such that life could evolve. They can hardly find themselves on a planet on which the past climate was such that their ancestors died. A planet capable of hosting complex life might be a one-in-a-trillion fluke; but if there are trillions of planets then that fluke is going to happen. Goldilocks not Gaia; luck not life.

In Lucky Planet Waltham discusses a number of ways in which Earth might be special, but the most interesting discussion (to me, at least) was the influence of the Moon. Like most people with an astronomy background I believed that the Moon played a large role in stabilising the Earth’s spin axis. Waltham shows that the true story is more complicated than that. It turns out that … but, no, you should get the book and find out for yourself.

The only criticism I have of the book is that, at two pages, the Further Reading section is rather sparse. The book is aimed at a popular audience, so I can understand the reasons for having only a short section bibliography, but I would have appreciated more detailed references. (The author does link to his website, however, and you can find more technical references there.) That minor quibble aside – thoroughly recommended!

When you look up at the stars on a clear night sky it’s difficult not to wonder whether there are conscious, sentient, intelligent beings somewhere out there. And if you accept for a moment their existence, a host of questions follow. What do they look like? Do they create art or make music? Do they believe in god? What is their understanding of space and time? Then another question intrudes, or at least it does whenever I find myself pondering these things: what if they aren’t there? What if humanity is the only species in the universe capable of wondering whether it is alone?

In Five Billion Years of Solitude: the Search for Life Among the Stars, Lee Billings records his discussions with a number of luminaries in the nascent field of astrobiology. Along the way Billings provides crystal-clear descriptions of the science behind the quest for exoplanets, the difficulties of SETI research, the technology with which we might find biosignatures on distant worlds, and much else besides. He gives a masterclass in how to make complex ideas accessible.

But the book is much more than that. The scientists he interviews are driven by competition, ego, rivalry – as are many achievers in various activities; science is not unique in this regard – but they are also united by an overwhelming desire to know whether we are alone in the universe. This makes their accounts both exciting and bitter-sweet. The final chapter in particular, which tells the story of how Sara Seager, a professor of planetary science and physics at MIT, came to find herself searching for the first truly Earthlike planet, is achingly beautiful.

Seager, Marcy or Kasting, or one of the many other scientists interviewed by Billings, might one day find life out there. But it’s entirely possible that they’ll fail (in my view it’s almost certain that they will fail). Earth, this precious blue marble, might be home to the only intelligent life in the universe. Reading Five Billion Years of Solitude will give you an appreciation of the true poignancy of that thought.

The Kepler mission team announced exciting new results on 7 January 2013, at the 221st meeting of the American Astronomical Society. As I’ve explained in other posts (such as this one here), the team aims to detect exoplanets by staring at more than 150,000 stars in a fixed part of the sky. The Kepler telescope looks for regular dips in the brightness of these stars, variations that might be caused by the presence of a transiting planet.

The technique used by the Kepler team has been tremendously successful. On 7 January Christopher Burke, a Kepler scientist at the SETI Institute, announced the discovery of 461 new planetary candidates. So, as of the time of writing, Kepler has found 2740 potential planets orbiting 2036 stars. This is a hugely impressive number, when you consider that it wasn’t such a long time ago that people were debating whether exoplanets existed at all and, if they did, whether it would be possible to detect them.

The sizes of the planetary candidates discovered by Kepler (Credit: NASA/Kepler)

The AAS presentation that really caught the attention of the world’s press, however, was that given by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. Fressin has tried to estimate the fraction of stars that possess of Earth-sized planets, based on the Kepler data. Kepler will inevitably see only a small fraction of exoplanets because the transit technique only picks up those planetary systems in which the orbital plane is more or less side on to our view. If the orbital plane is slanted by more than a few degrees to our line of sight, the planets we won’t see the planets transit and there’ll be no drop in brightness. On the other hand, Kepler is seeing regular brightness fluctuations that are not due to transits; for example, a non-variable star might be extremely close in our line of sight to a regular variable, which would give a similar signal to a transit. After Fressin had corrected for both these effects he arrived at the following estimate: one in six stars host an Earth-sized planet. If that is indeed the case, there are about 17 billion Earth-sized planets in our Galaxy and it’s certain that some of these will be in the habitable zone.

If we’re looking to explain the Fermi paradox, we now know that we can’t invoke a paucity of Earths!

One of the more outlandish proposals I discussed in Where is Everybody? was the “planetarium hypothesis” – an idea put forward by Stephen Baxter, one of the world’s foremost science fiction writers. Baxter argued that one popular idea in science fiction – that we live inside an artificial reality constructed by beings far in advance of ourselves – could actually be tested by experiment: even the most advanced civilisation would have constraints placed upon it by the laws of physics, and these constraints could be tested by experiment. If it turned out that we were living inside an artificial reality, that we were mere simulations in a computer program constructed by some mighty alien intelligence … well, that would be the Fermi paradox solved!

I didn’t take the planetarium hypothesis seriously, of course, but some people do. Nick Bostrom, for example, has taken the idea further. Bostrom is a professor of philosophy at Oxford University, and he has done some serious analysis of the notion that we are living in a computer simulation. Bostrom’s work is real philosophy, and mind-bending stuff. I urge you to visit his website, and think about his ideas: they are fascinating.

Three physicists who have clearly been influenced by Bostrom’s work are Silas Beane, Zohreh Davoudi and Martin Savage and last week they published a paper on arXiv (entitled “Constraints on the universe as a numerical simulation“) that examines some aspects of this seemingly bizarre notion. Beane, Davoudi and Savage are lattice gauge theorists, which means that they create a simulated toy “universe” in order to study quantum chromodynamics, the fundamental force that governs the interactions of quarks and gluons. They’ve extrapolated the current trends in computational requirements for lattice QCD, and examined the notion that our own universe is a numerical simulation on a spacetime lattice. They then ask the question: would there be any observable consequences if we were living in such a simulation?

Read their paper for the details. But their final sentence sums it up: “assuming that the universe is finite and therefore the resources of potential simulators are finite, then a volume containing a simulation will be finite and a lattice spacing must be non-zero, and therefore in principle there always remains the possibility for the simulated to discover the simulators.”

Stephen Baxter showed how a K3 civilisation could create a perfect simulation with a radius of about 100AU. Well, Voyager 1 has already passed beyond that distance (as I write, it’s at a distance of 122.64AU) and it didn’t bump into a metal wall painted black. So we know – by experiment! – that we don’t live in a K3 civilisation’s perfect simulation (though of course the Voyager result hasn’t ruled out the possibility that we live in an imperfect simulation; the boundary of an imperfect simulation can be much further away). The work by Beane, Davoudi and Savage provides us with other tools for testing whether we inhabit a simulation.

Solution 41 in Where is Everybody? is entitled “Earth’s system of plate tectonics is unique”. Why should volcanism and plate tectonics have anything to do with life? Well, I don’t plan to go into detail here – you can always read the book – but there are various ways in which plate tectonics might play a role in the emergence of life and the long-term viability of a planet. In Earth’s case, for example, volcanoes vomit lots of CO2: this is a greenhouse gas, which could have kept the surface of our young planet warm enough to allow water to remain in the liquid phase. As Earth aged, volcanism and plate tectonics played a key role in the carbon cycle: plate tectonics captures CO2 and takes it into the planet’s interior while volcanism releases CO2 back into the atmosphere. This carbon cycle, so it’s believed, plays a role in stabilising Earth’s climate – and a stable climate in turn may have been necessary for the development of complex life. And Earth’s magnetic field, which is driven by a rotating liquid metallic core, protects the atmosphere from high-energy cosmic radiation.

I doubt that in a universe as large as ours there is only one planet, Earth, that possesses plate tectonics. Recent work, however, suggests that the phenomenon may not be common – at least not on “super Earths” (rocky planets with a mass between 2-10 times that of Earth).

A team led by Vlada Stamenkovic has studied how temperatures within a super Earth are likely to change over time. There are some uncertainties in the work, but in the scenarios that Stamenkovic’s team studied it turned out that super Earths cool slowly: plate tectonics is unlikely to occur and a slow planetary-core formation (or even no core formation at all) means that magnetic fields are likely to be absent.

I’ve heard lots of people say recently that rocky super Earths, orbiting in the habitable zone, might be suitable places for life to originate and prosper. I think it much more likely that those planets are lifeless.

For full details of the work, see: Stamenkovic, V., Noak, L., Breuer, D. and Spohn, T. (2012) The influence of pressure-dependent viscosity on the thermal evolution of super-Earths. Astrophysical Journal, 748: 41.

News of the death of Neil Armstrong on 25 August 2012 brought back so many vivid memories of the Moon landing – as I’m sure it did to hundreds of millions of people around the world. I remember being woken by my father to watch the grainy television images of Armstrong landing Eagle on the lunar surface, and his subsequent historic step. Although I was young, I knew even then that I was watching something historic. What I didn’t appreciated until later was the incredible coolness under pressure that Armstrong displayed in landing Eagle successfully; what I didn’t appreciate until much, much later was Armstrong’s integrity in refusing to cash in on his status as the world’s most famous person. (It’s said that Armstrong was aloof, reclusive, and unwilling to engage with the public. That’s not the case. My daughter’s primary school, for instance, has a plaque signed by Armstrong. He just decided to use his time in the way that he thought best, that’s all.) Neil Armstrong was a true hero.

Neil Armstrong (Credit: NASA)

A total of 12 astronauts, including Armstrong, have set foot on the Moon. Following Armstrong’s death, only 8 of those 12 astronauts survive. The youngest of the survivors is Charles Duke, and he is 76. Without wishing to be ghoulish, it won’t be many more years before there is no living human being who has set foot on the Moon. The sobering fact is that, since Eugene Cernan shook the Moondust from his boots in December 1972, no human has returned to our sister planet. It is hard to imagine anyone returning to the Moon anytime soon.

Does that matter?

I believe it does. To strive, to seek, to find… I believe those are important qualities for humankind to display. Science can certainly be done by unmanned probes and robots (as we are seeing right now with the wonderful Mars Curiosity rover), but if mankind itself chooses to stay at home on Earth and leave the striving to probes then I believe we are heading for trouble. The rich, developed countries possess an economic system that appears to be little more than a glorified Ponzi scheme; we seem hell-bent on burning up the planet; we aren’t developing new sources of energy at the rate our ever-increasing population requires. Earth is our spaceship and we are without lifeboat – and, unfortunately, it seems our species doesn’t want to build a lifeboat.

In Where Is Everybody? I argued that resolutions of the Fermi paradox that depend on “sociological” explanations – such as supposing that all extraterrestrial civilisations perish in some global catastrophe (nuclear warfare, biological warfare, civilisation-induced climate change … take your pick) – are unconvincing. That’s still the case. Nevertheless, maybe it’s because I’m in a gloomy mood following news of Armstrong’s death, but I do increasingly wonder whether one of those global catastrophes will end humanity’s chance of signalling its presence to the rest of the Universe.

Solution 16 in If the Universe is Teeming with Aliens…Where is Everybody? is entitled “They are signaling, but we do not know how to listen”. In that section I discuss a solution to the Fermi paradox that some scientists have occasionally proposed: extraterrestrial civilizations are out there, and sending signals, but we don’t hear them because we’re listening in the wrong way. Perhaps, the argument goes, we’re listening for electromagnetic signals when we should be listening for modulated neutrino beams, gravitational waves, or cosmic rays. In particular, the possibility of using neutrinos for interstellar communication was proposed by Mieczyslaw Subotowicz as long ago as 1979 (in the paper “Interstellar communication by neutrino beam”, which appeared in volume 6 of Acta Astronoautica; see pages 213-220).

When I wrote Where is Everybody?, neutrino communication was not possible and it seemed to me unlikely that it would ever be possible in my lifetime. How quickly technology progresses! A recent paper by Daniel Stancil and colleagues (Demonstration of communication using neutrinos) reported on how a communications link was established between the NuMI beam line and the MINERvA detector (both at Fermilab). OK, so the distance involved here are not on an interstellar scale (in fact, the separation was only 1035m); and the transmission figures aren’t stunning (they achieved a decoded data rate of 0.1 bits per second, with a bit error rate of 1%). But it’s a start! Ten years ago this couldn’t be done; in ten years time this will be routine.

The work by Stancil and his colleagues will eventually have applications, particularly in scenarios where communication using electromagnetic waves is difficult or impossible. (The 1035m over which the neutrino beam allowed communication to take place included 240m of solid earth: this was direct communication, rather than boring a tunnel that could house a optical fiber that could then carry an electromagnetic. Or consider the case of communication with a submerged nuclear submarine: seawater is opaque to short-wavelength electromagnetic radiation, which is why submarines must come close to the surface and float a communications line, but neutrinos go straight through water – as they do with everything else.) So neutrinos may have a role in future communications technology. But will they play a role in communicating over interstellar distances?

I remain unconvinced that any extraterrestrial civilizations would choose to broadcast a signal using neutrinos. First, electromagnetic radiation is a faster signal carrier than a neutrino beam. (Despite the recent story about those OPERA neutrinos, we know that neutrinos don’t travel faster than light.) Second, its vastly easier to generate modulated electromagnetic radiation than it is to generate a modulated neutrino beam. Third, its vastly easier to detect modulated electromagnetic radiation than it is to detect a modulated neutrino beam. Fourth, because we share the same universe and are subject to the same laws of physics, we can be reasonably sure than any extraterrestrial civilization will know all of the above – they’ll know that we know that they know all this.

If there are any extraterrestrial civilizations out there trying to contact others in the universe, surely they’ll be using electromagnetic radiation. Won’t they?